Understanding the benefits of Al3+-doping on NaSICONs explained through an out-of-the-scheme isovalent substitution of Fe3+ in Na3Fe2(PO4)3 series

Abstract

NaSICON fast-ionic conductors have been largely considered as a viable solution for the development of energy storage units. Chemical doping is a popular technique to improve their electrochemical performance, and the widely accepted criterion is an aliovalent substitution, to increase the cationic density, and/or an isovalent substitution with larger species, to enlarge the bottleneck, as widely speculated for the LATP series. However, this study demonstrates through electrochemical impedance spectroscopy that the ionic conductivity of a Na3Fe2(PO4)3 NaSICON isovalently doped with smaller Al3+ ions to form a Na3AlxFe2-x(PO4)3 NaSICON series can be improved up to 4 times. We resorted to X-ray diffraction and ab initio simulation to show that the NaSICON bottlenecks are enlarged despite the lattice shrinkage and the long-range diffusion path is unlocked for x ≥ 0.5. Moreover, the Al-doping sizeably increases the cohesive energy of the monoclinic lattice, thus stabilizing it. As a result, the cyclic voltammetry and galvanostatic charge-discharge tests of the half-cell battery with Na3AlxFe2-x(PO4)3 as the composite cathode showed better capacity and no decay. We believe that this out-of-the-scheme work will clarify the effects of Al-doping on NaSICON compounds and at the same time contribute to reviving the interest around this highly-sustainable Fe3+-based NaSICON.